1. Field
This disclosure relates to variable gain amplifier circuits.
2. Description of Related Art
Differential variable gain amplifiers (VGA) circuits are commonly used in a number of applications. As some examples, VGA circuits are used in automated test equipment, cellular system base stations and terrestrial links in satellite systems, among any number of other applications. In current embodiments of such VGA circuits, a gain of a specific VGA circuit (an amount of amplification of an applied input signal achieved) is controlled by varying the amount of current that flows through a differential pair of transistors that is included in the VGA. This current is referred to as Io, or “tail current.” As the tail current is increased, the gain of the VGA circuit increases. Likewise, as the tail current is decreased, the gain of the VGA circuit decreases.
One important operating characteristic of such VGA circuits is linearity. Linearity is a measure of the variation of output signal strength in direct to proportion to input signal strength. A standard measure of the linearity of a VGA circuit is what is referred to as the third input intercept point (IIP3). The IIP3 of a VGA circuit is the output power amplitude at which the output of a fundamental input signal and the output power amplitude of a third harmonic of the fundamental input signal have equal magnitude. The greater the value of IIP3 is for a particular VGA circuit, the greater the linearity of that circuit is.
One drawback of current VGAs is that IIP3 varies as result of controlling the gain of the VGA. Specifically, as the tail current is varied to adjust the gain, IIP3 also varies. As with the gain of a VGA, the value of IIP3 for a given VGA circuit increases as the tail current (and gain) is increased and decreases as the tail current (and gain) is decreased. Such a relationship is undesirable as the linearity of the VGA will be reduced when the gain of the circuit is adjusted to operate at lower levels. This reduction in linearity reduces the range of amplitudes of input signals that the VGA is able to receive and reliably process. Therefore, alternative approaches for implementing VGA circuits are desirable.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.